As is known in the art, a double heterojunction (DH) Gallium Nitride (GaN) High Electron Mobility Transistor (DH-HEMT) has a back-barrier providing charge confinement in the gate channel thereby improving gate control resulting in lowering off-state conduction and allowing for improved gain performance at higher voltage operation. See for example, an article entitled AlGaN Barrier/GaN channel/InGaN Back-barrier/GaN Buffer, T. Palacios, A. Chakraborty, S. Heikman, S. Keller, S. P. DenBaars, and U. K. Mishra, IEEE Electron Device Letters Vol. 27, 2006, pp. 13-15.
Typically this back barrier consists of an Aluminum Gallium Nitride (AlGaN) layer with relatively low Aluminum (Al) mole-fraction (typically 3%˜8%). However, because of polarization, the channel charge (also called 2 Dimensional Electron Gas or 2-DEG) in the DH-HEMT is lower compared to a comparable HEMT structure without back-barrier, thereby impacting maximum current. To compensate for the reduced 2-DEG and current, the Al mole-fraction and/or the thickness of the top-barrier layer is increased. However, this brings several drawbacks including increased strain in the semiconductor structure leading to performance degradation.